The user Laserschwert of lucasforums.com has done a great job of scanning and retouching some of the game-boxes used for the classic LucasArts adventure games like: Monkey Island, Indiana Jones, Sam & Max and Day of the Tentacle series. See this thread over at lucasforums.com where you could download these posters.
I’m thinking of getting some of these posters printed out myself, once I find a great (online) printshop here in Norway.

A poster of the original art used on the back of the Monkey Island 2: LeChuck’s Revenge game was recently sold on eBay by the original artist, Steve Purcell.
It would’ve been nice if the buyer could have scanned this poster and post it online so that others could enjoy it. But since he payed $4550 USD (about 30 000 NOK) for the painting, I guess he would not do that. :-P

Some non-retouched scans of these game-boxes are available at the mojoart.mixnmojo.com site, like the backside of Monkey Island 2, as I wrote about above. Unfortunately this version has too much text, screen shots and other stuff over the original painting to be able to use it as a poster like that sold on eBay.